The present invention relates to electric meters and, more particularly, to apparatus for developing a driving voltage for meter accessories.
Conventional electro-mechanical electric meters employ a conductive metal disk rotated as the rotor of a small induction motor by interaction with opposed voltage and current stators. The rotational torque experienced by the disk is proportional to the voltage applied to the load multiplied by the current consumed by the load; that is, the power consumed by the load. Disk rotation is magnetically resisted in proportion to its rotational speed. Thus, the disk speed is proportional to the power consumed by the load. Each rotation of the disk represents a predetermined increment of energy consumed. The rotations of the disk are accumulated over time in a mechanical or electronic accumulator, or register, for reading by the utility supplying the power for billing purposes.
For some purposes, it is desirable to produce a small amount of auxiliary power within the electric meter for driving accessory devices such as, for example, an indicator which responds to excitation of the core material in a voltage stator. In meters having two or three voltage stators, an independent indicator is desirable for each voltage stator to provide assurance that proper excitation exists.
Voltage stators conventionally employ a core consisting of E-shaped laminations. A voltage stator winding is wound about the center leg of the core. U.S. Pat. No. 3,815,027 discloses a small winding disposed in the auxiliary gap between the center leg and one of the side legs of the core for developing a small voltage. This small voltage may be used, for example, to illuminate a light-emitting diode. This technique tends to unbalance the flux field of the voltage stator by as much as, for example, about seven percent at light loads. Such an imbalance tends to upset the light load adjustment of the meter. Thus, the small winding cannot be installed after manufacture without repeating the light load adjustment. If late addition of such a small winding is contemplated, the adjustment range built into the light load adjustment must be made sufficiently broad not only to accommodate normal manufacturing tolerances, but also to include additional range to overcome the imbalance imposed by the small coil in the auxiliary gap. When the range is thus increased, the resolution of adjustment is conventionally degraded. That is, when a screw-type adjustment is provided, if the full adjustment range consists of two screw turns, if a total adjustment range of ten percent is required, then each turn provides an adjustment of five percent. If an additional range of ten percent must be provided to prepare for a possible late addition of a coil in the auxiliary gap, each screw turn produces a ten-percent change in light load adjustment. Thus each incremental turn of the adjustment screw has twice the effect in the latter case than in the former. Consequently, precise light-load adjustment is more difficult to accomplish.